NASA Evaluates Compact Synthetic Aperture Radar

PASADENA, Calif. - NASA is evaluating a compact L-Band synthetic aperture
radar for potential use on unmanned aircraft. The sensor detects and measures
small changes in Earth's surface of geophysical interest, such as volcanoes,
earthquake faults, landslides and glaciers.

NASA's Jet Propulsion Laboratory, Pasadena, Calif., and Dryden Flight Research Center,
Edwards, Calif., are partnering in development of the Unmanned Aerial Vehicle Synthetic
Aperture Radar (UAVSAR). A modified NASA Gulfstream III aircraft carries the JPL-developed
radar in a custom-built pod under the aircraft's fuselage during its development phase.

"The system is an imaging radar. It is like a camera that uses microwaves to make an image,"
said Scott Hensley, chief scientist for the project at JPL. "It collects data to measure
the deformation of Earth's surface, for example from an earthquake or volcano. This will
help us better understand how earthquakes and volcanoes work. The way we do that is through
the signatures of how they deform Earth's surface."

"The UAVSAR is a flying test bed for developing the tools and technologies for future space-based
radars," said Robert Smith, UAVSAR program manager for NASA's Earth Science Technology Office.
"Once operational, it will also be a powerful airborne instrument providing calibration data
and rapidly repeated images scientists need to augment data obtained from satellites."

During these validation flights, the aircraft is using a technique known as repeat pass
interferometry that requires the aircraft to fly each pass as close to the original flight
line as possible. For the experiment, two data passes, flown from minutes to months apart,
are compared to examine changes in Earth's surface.

To ensure the accuracy of the flight paths, a precision autopilot designed by engineers
at Dryden has been installed on the test aircraft. The autopilot guides the aircraft using
the differential Global Positioning System and the aircraft's inertial navigation system to
repeat the flight path to an accuracy of within 4.6 meters (15 feet). With the precision
autopilot engaged, the synthetic aperture radar will be able to acquire repeat pass data
that can measure changes with a resolution measured in millimeters.

"We're very pleased with the performance of the Platform Precision Autopilot, and feel
confident that this new system, along with the structurally modified G-III aircraft,
is ready to fully support UAVSAR interferometry studies," said Frank Cutler, project
manager for the Gulfstream III UAVSAR flight testing at NASA Dryden.

During the 1990s, scientists used NASA's DC-8 airborne science laboratory to collect
data with the airborne synthetic aperture radar (AIRSAR) system, also developed at JPL.
The UAVSAR instrument currently under development has benefited from technology
advancements in the 20 years since AIRSAR was built.

The prototype UAVSAR is smaller, fitting into a pod about three meters (10 feet) in length.
The radar pod is a self-contained instrument package that requires only electrical power
from the aircraft. The instrument has its own navigation system consisting of a high-accuracy
inertial navigation unit and a differential GPS developed at JPL that provides the aircraft's
location to an accuracy of less than one meter (three feet). The radar's electronically
steered antenna compensates for aircraft attitude changes as the radar makes repeated
passes over areas of interest.

The pod also contains a two-terabyte recorder to store the large amounts of data generated
by the radar. The pod is designed to operate autonomously by loading commands to the system's
computer prior to flight. Along with the autopilot, the design of the pod allows it to be flown
on a variety of aircraft, including unmanned aircraft.

The sensor is currently undergoing a one-year development and test period to improve robustness
and validate its ability to meet the science objectives. The UAVSAR will be extensively tested
through 2008, after which it will become a community science tool for NASA.

NASA's Science Mission Directorate funded development of the UAVSAR. JPL is managed for NASA
by the California Institute of Technology in Pasadena